Which of the following is the primary mechanism by which insulin suppresses glucagon secretion from pancreatic alpha cells?

Explanation

## Mechanism of Insulin-Mediated Glucagon Suppression **Key Point:** Insulin suppresses glucagon secretion primarily through a **paracrine mechanism** involving somatostatin (SST), not direct inhibition of alpha cells. ### Physiological Pathway 1. **Hyperglycemia** → Insulin secretion ↑ 2. **Insulin** → Stimulates delta cells (δ-cells) in pancreatic islets 3. **Delta cells** → Release somatostatin (SST) 4. **Somatostatin** → Diffuses locally and inhibits alpha cells (α-cells) 5. **Result:** Glucagon secretion ↓ **High-Yield:** This is a **paracrine interaction** — SST acts on neighboring alpha cells, not systemically. The three pancreatic hormones (insulin, glucagon, SST) form a regulatory triangle: | Hormone | Secreted by | Stimulates | Inhibits | |---------|-------------|-----------|----------| | Insulin | β-cells | δ-cells (SST) | α-cells (via SST) | | Glucagon | α-cells | β-cells (insulin) | δ-cells (SST) | | Somatostatin | δ-cells | — | α-cells, β-cells | **Clinical Pearl:** This paracrine regulation ensures that when glucose is high (insulin ↑), glucagon is suppressed, preventing futile glucose cycling. Conversely, during hypoglycemia, low insulin allows alpha cells to secrete glucagon unopposed. **Mnemonic:** **"SST Stops Glucagon"** — Somatostatin is the paracrine brake on alpha-cell glucagon release.